1. Field of the Invention
This invention is directed to a new process for strengthening geological formations in underground workings or mines, in particular in coal mines, with reaction mixtures based on organic polyisocyanates and polyhydroxyl compounds which contain special esters of (i) polymerized unsaturated fatty acids or mixtures thereof with monomeric fatty acids and (ii) low molecular weight, polyhydric alcohols as compounds for regulating the foaming process.
2. Description of the Prior Art
The strengthening of geological formations below ground, particularly in coal mining, by means of foamable polyurethanes which harden within the formation is already known (see e.g. The Journal Gluckauf (1968), pages 666 to 670; Gluckauf (1977), pages 707 to 711; and Bergbau (1977) pages 124 to 129, DE-PS 1,129,894, DE-PS 1,758,185, DE-PS 1,784,458, DE-PS 2,436,029, DE-PS 2,623,346 and DE-PS 3,139,395). Such mixtures which react to form polyurethanes are also used for producing barriers which seal off water and/or gas.
The above-mentioned process, which is regarded as state of the art, is generally carried out as follows: the two components of the reaction, i.e., the polyisocyanate and the polyhydric alcohol (polyhydroxyl component), are carried separately to a point in front of the bore hole where they are continuously combined in a static mixer and forced through the bore hole into the rock formation where the mixture foams up and hardens.
Geological formations which are to be strengthened on this basis, in particular layers of rock below ground, vary considerably in their water content. This means that if reaction mixtures essentially containing polyisocyanates and the conventional polyether polyols if polyurethane chemistry are used, the polyisocyanate component reacts with the water present in the formation to an extent which cannot be controlled. If the water content in the layers of rock is high, the reaction may result in a lightweight polyurea foam which does not provide sufficient strengthening of the rock formation. To ensure adequate and homogeneous strengthening of the rock formation, it is necessary to achieve controlled and homogeneous formation of foam regardless of the water content of the formation. It should also be possible to predetermine the properties of the resulting foams by suitable choice of the composition of the reaction mixture regardless of the water content. It has been shown in the past that the use of castor oil in addition to the known polyether polyols in the polyol component is a first step in this direction. The addition of castor oil, generally in quantities of about 5 to 20% by weight, based on the polyol component, reduces the tendency of unwanted side reactions between polyisocyanate and water. This suppresses excessive foaming of the polyurethane resins, especially in contact with the water present in the formation and, thus, prevents the loss of mechanical properties which would otherwise occur.
It has been found in practice, however, that it would be desirable to reinforce the ability of castor oil to suppress the unwanted reaction between isocyanate and water in order to produce a very homogeneous polyurethane resin which will ensure uniform strengthening of the rock formation.
Another disadvantage of using castor oil is that it is a natural product which is therefore subject to considerable variations in its properties. Further, since castor oil is required to be used in large quantities as mentioned above, it constitutes a considerable cost factor because the cost of this natural product is subject to wide fluctuations and may even be greater than the cost of conventional polyether polyols.
It is therefore an object of the present invention to provide a new process for strengthening geological formations which would be more capable of suppressing the unwanted reaction between isocyanate and water and the resulting uncontrolled foaming up of the reaction mixture than has previously been possible in the art.
This object has been achieved by the process according to the invention described below, in which certain esters of polymerized unsaturated fatty acids or mixtures thereof are used in combination with monomeric saturated or unsaturated fatty acids and low molecular weight alcohols. It has surprisingly been found that by using these esters it is possible not only to exceed the above-mentioned advantageous property of castor oil but also to substantially improve the mechanical properties of the polyurethane products. The addition of esters of polymerized fatty acids and low molecular weight polyols prevents the formation of lightweight foams even in rock layers with a high water content so that the process according to the invention provides uniformly expanded polyurethanes with excellent mechanical properties.
The finding that esters of polymerized unsaturated fatty acids and low molecular weight polyols prevent the formation of lightweight foams is surprising since the polymerized unsaturated fatty acids from which these esters are obtained act as stabilizers for polyurethane foams and therefore have exactly the opposite effect. Thus according to U.S. Pat. No. 3,476,933, di- and trimerized fatty acids are used for the production of polyurethane foams with large cells. Unesterified polymerized fatty acids have no influence on the reaction between isocyanate and water in the process according to the invention and do not prevent the uncontrolled foaming up of the reaction mixture.